In the production of polymer contact lenses, a polymerizable lens precursor composition is polymerized to form a contact lens product that is further processed to form a hydrated contact lens. Lenses, such as hydrogel contact lenses and silicone hydrogel contact lenses, have been made from a variety of different materials by that method. Conventional hydrogel contact lenses include contact lenses made from materials having a US Adopted Name (USAN) such as polymacon, tetrafilcon, ocufilcon, vifilcon, etafilcon, omafilcon, alphaphilcon, nelfilcon, hilafilcon, tefilcon, or vasurfilcon, for example. Frequently, conventional hydrogel contact lenses are the polymerized product of a lens precursor composition containing hydrophilic monomers, such as 2-hydroxyethylmethacrylate (HEMA), methacrylic acid (MA), methyl methacrylic acid (MMA). N-vinyl pyrrolidone (NVP), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), and combinations thereof. The precursor compositions also frequently contain one or more catalysts and crosslinking agents.
Silicone hydrogel contact lenses are polymerized contact lenses that include a silicone component. For example, silicone hydrogel contact lenses can be formed by polymerizing a lens precursor composition that contains a silicone-containing monomer, oligomer, macromer, polymer, and the like, in addition to other lens forming materials. Examples of publicly available silicone hydrogel contact lenses include contact lenses formed from materials having a USAN of balafilcon A (PURE VISION, Bausch & Lomb), lotrafilcon A (NIGHT & DAY, CIBA Vision), lotrafilcon B (O2OPTIX, CIBA Vision), galyfilcon A (ACUVUE ADVANCE, Vistakon), senofilcon A (ACUVUE OASYS, Vistakon), and comfilcon A (BIOFINITY, CooperVision).
The polymerization or curing of the lens precursor composition can be accomplished by exposing contact lens mold assemblies containing the lens precursor composition to ultraviolet radiation or heat, in a curing oven for example. The type of radiation used to polymerize the precursor composition is often dependent on the chemical formulation. When a chemically inert curing process is desired so that reactive oxygen does not affect the polymerization of the precursor composition, contact lens mold assemblies are treated and/or receive the polymerizable composition under chemically inert atmospheric conditions, for example under a nitrogen blanket.
Thermal curing of the lens precursor compositions in the contact lens mold assemblies can in principle take place in a normal or non-chemically inert atmosphere since the polymerizable lens precursor composition was dispensed in a chemically inert atmosphere, but in practice, many of the contact lenses produced in a normal atmosphere are rejected for various faults, many of which arise from problems resulting from volatile materials that are produced during the polymerization.
In thermal curing procedures for hydrogel contact lenses, volatiles released from the lens precursor composition as it polymerizes become a significant issue, especially from a regulatory viewpoint. Volatile by-products of the curing accumulate within the curing oven and must be removed, either by manual cleaning (which is usually impractical as it requires use of the oven to stop to allow the cleaning) or by using scrubbing filters. For example, activated carbon filters are often used to scrub volatile by-products resulting from the curing process; however, that can cause problems as contact lens manufacture requires a very clean environment, and activated carbon filters are usually not clean. Standard condensation filters are used to remove generated moisture.
The thermal curing process can involve altering the temperature of the curing chamber or curing zone of a batch-type curing oven several times during the curing. For example, the curing chamber of the oven may be kept at a constant first temperature of X degrees for A minutes, then heated to Y degrees and kept at that temperature for B minutes, then cooled to X degrees and kept at that temperature for A minutes. The different curing temperatures and times relate to the chemical formulations used to make the contact lenses. Therefore, the actual values described above are represented by variables for purposes of convenience. Other curing profiles are also possible.
Commercial-scale contact lens production requires very large numbers (typically tens if not hundreds of thousands) of contact lenses to be produced each day. When batch ovens are used, achieving those numbers requires several batch ovens of the type described above to be operated in parallel. However, each batch oven tends to provide a slightly different thermal performance. Also, the inert environment is difficult to control in a batch oven curing process, again leading to different oven performances. Those different performances make it difficult to control the manufacturing process to minimize the fraction of contact lenses that have defects that result in them being rejected.
There remains a need for new systems and methods for producing contact lenses, including hydrogel contact lenses and especially silicone hydrogel lenses, which quickly produce large numbers of contact lenses while reducing problems associated with existing methods and systems. For example, there is a need for new systems and methods to quickly produce in a chemically inert and volatile-free atmosphere large numbers of contact lenses having improved uniformity in their properties.